Hydrocarbons are essential in modern life and used as fuel and raw materials, including the chemical, petrochemical, plastics, and rubber industry. Fossil fuels such as oil and natural gas are composed of hydrocarbons with a specific ratio of carbon to hydrogen. In spite their wide application and high demand, fossil fuels also have limitations and disadvantages in the view of being a finite resource and their contribution to global warming if they are burned.
Research on alternative fuels was mainly started due to ecological and economical considerations. Among the alternative fuels, dimethyl ether (DME), which is recently discovered as a clean fuel, can be synthesized from synthetic gas that was generated from different primary sources. These primary sources can be natural gas, coal, heavy oil, and also from biomass. Up to now, only two DME synthesis procedures from synthesis gas have been claimed, whereby one is the traditional methanol synthesis, followed by a dehydration step and the other is a direct conversion of synthesis gas to DME in one single step.
Recently, attention has been directed towards the direct synthesis of dimethyl ether from synthesis gas, using a catalytic system that combines a methanol synthesis catalyst and a catalyst for dehydration of said alcohol. It was confirmed on the basis of experimental studies that both, the stage of methanol synthesis and the stage of methanol dehydration, could be conducted simultaneously on one appropriate catalytic system. Depending upon the applied synthesis gas the catalyst might additionally show water gas shift activity.
Most known methods of producing methanol involve synthesis gas. Synthesis gas is a mixture of mainly hydrogen, carbon monoxide and carbon dioxide, whereby Methanol is produced out of it over a catalyst.CO+2H2CH3OH
In a following step Methanol can be converted into DME by dehydration over an acidic catalyst.2CH3OHCH3OCH3+H2O
In the direct DME production there are mainly two overall reactions that occur from synthesis gas. These reactions, reaction (1) and reaction (2), are listed below.3CO+3H2CH3OCH3+CO2  (1)2CO+4H2CH3OCH3+H2O  (2)
Reaction (1) occurs with the combination of three reactions, which are Methanol synthesis reaction, Methanol dehydration reaction, and water gas shift reaction:2CO+4H22CH3OH (methanol synthesis reaction)2CH3OHCH3OCH3+H2O (methanol dehydration reaction)CO+H2OCO2+H2 (water gas shift reaction)
The reaction (1) has a stoichiometric ratio H2/CO of 1:1 and has some advantages over reaction (2). For example reaction (1) generally allows higher single pass conversions and less energy-consuming in comparison to the removal of water from the system in reaction (2).
Methods for the preparation of dimethyl ether are well-known from prior art. Several methods are described in the literature where DME is produced directly in combination with methanol by the use of a catalyst active body in both the synthesis of methanol from synthesis gas and methanol dehydration. Suitable catalysts for the use in the synthesis gas conversion stage include conventionally employed methanol catalyst such as copper and/or zinc and/or chromium-based catalyst and methanol dehydration catalyst.
The document U.S. Pat. No. 6,608,114 B1 describes a process for producing DME by dehydrating the effluent stream from the methanol reactor, where the methanol reactor is a slurry bubble column reactor (SBCR), containing a methanol synthesis catalyst that converts a synthesis gas stream comprising hydrogen and carbon monoxide into an effluent stream comprising methanol.
Document WO 2008/157682 A1 provides a method of forming dimethyl ether by bimolecular dehydration of methanol produced from a mixture of hydrogen and carbon dioxide, obtained by reforming methane, water, and carbon dioxide in a ratio of about 3 to 2 to 1. Subsequent use of water produced in the dehydration of methanol in the bireforming process leads to an overall ratio of carbon dioxide to methane of about 1:3 to produce dimethyl ether.
Document WO 2009/007113 A1 describes a process for the preparation of dimethyl ether by catalytic conversion of synthesis gas to dimethyl ether comprising contacting a stream of synthesis gas, comprising carbon dioxide with one or more catalysts active in the formation of methanol and the dehydration of methanol to dimethyl ether, to form a product mixture comprising the components dimethyl ether, carbon dioxide and unconverted synthesis gas, washing the product mixture comprising carbon dioxide and unconverted synthesis gas in a first scrubbing zone with a first solvent rich in dimethyl ether and subsequently washing the effluent from the first scrubbing zone in a second scrubbing zone with a second solvent rich in methanol to form a vapor stream comprising unconverted synthesis gas stream with reduced content of carbon dioxide transferring the vapor stream comprising unconverted synthesis gas stream with reduced carbon dioxide content for the further processing to dimethyl ether.
Document WO 2007/005126 A2 describes a process for the production of synthesis gas blends, which are suitable for conversion either into oxygenates such as methanol or into Fischer-Tropsch-liquids.
The U.S. Pat. No. 6,191,175 B1 describes an improved process for the production of methanol and dimethyl ether mixture rich in DME from essentially stoichiometrically balance synthesis gas by a novel combination of synthesis steps.
In document US 2008/125311 A1 is a catalyst used for producing dimethyl ether, a method of producing the same, and a method of producing dimethyl ether using the same. More particularly, the present invention relates to a catalyst used for producing dimethyl ether comprising a methanol synthesis catalyst produced by adding one or more promoters to a main catalyst comprised of a Cu—Zn—Al metal component and a dehydration catalyst formed by mixing Aluminium Phosphate (AlPO4) with gamma alumina, a method of producing the same, and a method of producing dimethyl ether using the same, wherein a ratio of the main catalyst to the promoter in the methanol synthesis catalyst in a range of 99/1 to 95/5, and a mixing ratio of the methanol synthesis catalyst to the dehydration catalyst is in a range of 60/40 to 70/30.
The processes for the preparation of dimethyl ether according to the prior art bear the drawbacks that different steps have to be undergone to get an efficient DME production. Besides this, the catalyst used in the method known in prior art does not achieve the thermodynamic possibilities. Therefore it is still desirable to increase the yield of DME in the synthesis gas conversion.